1. Field of the invention
The present invention relates to an ignition timing control apparatus for an internal combustion engine using an ignition timing table, and especially to an electronic ignition timing control apparatus suitable for an automobile gasoline engine, which can be supplied with fuel having the different knocking-resistant characteristics.
2. Description of the prior art
In an internal combustion engine, such as a gasoline engine, it is preferable to make the engine operate at the ignition timing advanced almost up to the limit of occurrence of knocking, in the point of view of full use of its performance. If, however, the ignition timing is fixed at such a limit value, the stable operation can not be always obtained due to the small change in the engine characteristics or the variance in the quality of fuel, with the result that knocking often occurs and the performance of the engine falls.
The frequent occurrence of knocking reduces the span of life of an engine. In an extreme case, an engine comes to be broken down. Therefore the occurrence of knocking must be prevented to the utmost, especially in an engine with a so-called turbo-charger. Then, there is recently used a knocking control system, in which the occurrence of knocking is detected and the ignition timing is controlled by the feed-back control on the basis of a knocking detection signal, whereby it becomes possible to always operate an engine at the ignition timing near the limit of occurrence of knocking.
In the conventional knocking control system, the occurrence of knocking was detected by a knocking detector disclosed in the laid-open Japanese patent application JP-A-58/217773 (1983), for example. According thereto, a sensor, which is a kind of an acceleration detector, is attached to an engine block to detect the vibration of an engine. Since the vibration of an engine includes harmonics extending over the considerably wide range of frequencies, components including only harmonics of frequencies, e.g., 6 to 8 KHz, which are said to be inherent to the knocking phenomenon of an engine, are extracted from the output signal of the sensor by a band-pass filter.
The output signal of the band-pass filter is amplified to form a vibration signal SIG having a predetermined level, on one hand, and is subject to the half-wave rectification and smoothed to form a background level signal BGL, on the other hand (cf. FIG. 1b). The thus obtained signals are compared with each other, and when the former signal exceeds the latter one, a pulse signal is produced. Therefore, a knocking detection signal of the knocking detector becomes a pulse train signal (cf. FIG. 1c), and the number of pulses within a constant period indicates the intensity of the occurring knocking. The constant period is defined by a reference signal REF generated at every predetermined rotational angle of an engine crank shaft (cf. FIG. 1a).
The conventional knocking control system carries out the following control operation by using the knocking detection signal as mentioned above. There is provided a counter, which counts pulses of the knocking detection signal and is cleared by the reference signal. Every time when the counter is cleared, the content thereof is taken, because the content just before the clear of the counter represents the intensity of knocking (cf. FIG. 1d).
The amount .theta..sub.R of an ignition delay angle in proportion to the detected intensity n of knocking is read out from an ignition control table, which is provided in advance. The aforesaid amount .theta..sub.R is called a proportional component. The ignition timing at that time, which is given as a basic ignition timing .theta.(N,L), is delayed by the proportional component .theta..sub.R to obtain a actual ignition timing .theta..sub.IGN. After that, the actual ignition timing .theta..sub.IGN is returned again toward the basic ignition timing .theta.(N,L) with time at a predetermined rate .theta..sub.I (cf. FIG. 1e). This rate .theta..sub.I is called an integration component. Therefore, an overall correction amount .theta..sub.KN is represented by (.theta..sub.R +.theta..sub.I). As a result, an ignition signal is produced at the time point which is delayed by an angle corresponding to .theta..sub.R from a time point according to .theta.(N,L) (cf. FIG. 1f).
According to this control system, when the occurrence of knocking is detected, the ignition timing is delayed at once in accordance with the intensity of the occurring knocking, whereby the occurrence of knocking is effectively suppressed, and an engine can show its full performance.
As is well known, however, the occurrence of knocking in a gasoline engine greatly depends on the octane value of fuel. Generally, knocking is easy to occur with low-octane gasoline and difficult to occur with high-octane gasoline. In order to effectively suppress the occurrence of knocking, therefore, an engine has to be controlled, taking account of the nature of fuel, such as octane value thereof.
Then, in a conventional engine control apparatus, it was supposed that an engine is operated with the constant nature of fuel, and various data necessary for the engine control, including data for controlling the ignition timing, were provided in advance on the basis of the supposed nature of fuel. If, therefore, an engine is supplied sometimes with low-octane gasoline and sometimes with high-octane gasoline, the precondition provided in view of the constant nature of fuel is destroyed and an undesirable influence appears on the performance and durability of the engine.
In addition thereto, there was also the limit in coping with uncontrollable factors, such as aged deterioration. From those reasons, the conventional engine control apparatus was difficult to always maintain the ignition timing at an appropriate value commensurate with the characteristics of an engine.
To solve the problem as mentioned above, there is proposed an ignition timing control apparatus, as disclosed in the laid-open Japanese patent application JP-A-62/58057 (1987), for example. According to this, there are prepared in advance a plurality of different tables for the ignition timing control, which have different data in response to the nature of fuel and the change in the engine characteristics. An adaptive ignition timing table is selected from among them in accordance with the degree of occurrence of knocking, and the ignition timing control is carried out by using the selected table.
In practice, however, there does not always appear the significant difference in the intensity of knocking occurring when an engine is operated with fuel of the different octane value. The intensity of knocking differs every engine because of the difference in the operational condition and the aged deterioration. Therefore, a threshold for judging the occurrence of knocking to accordingly select an adaptive one of ignition timing tables had to be set at a different level for every engine. It was difficult to determine a threshold level, which can be used in common for every engine. As a result, a common threshold level had to be set rather high so as not to erroneously judge the occurrence of knocking.
Further, in the prior art apparatus, no consideration was given to the ignition timing control at the time of switchover of an ignition timing table, with the result that knocking caused by the abrupt change of an ignition timing table could not be prevented from occurring. Moreover, since a threshold level was set rather high, there was the danger that knocking becomes always hard, when it occurs, whereby a serious damage is applied to an engine.
Additionally, if an ignition timing table for low-octane fuel is selected, notwithstanding an engine is actually supplied with high-octane fuel, the knocking scarcely occurs in the engine. In such a case, an ignition timing table was never switched over in the prior art apparatus, because it had no function for judging the octane value of fuel used. Therefore, the engine could not show its full performance commensurate with the fuel used at that time, as well as the effect of plural ignition timing tables specially provided could not be obtained.